Disable integrated vega 8 gpu

I have acer nitro 5 laptop.It has Ryzen 2500u cpu and RX560x gpu. ryzen has a integrated gpu vega 8.Ram is 8 GB but it shows only 6.90 GB usable.due to integrated vega 8 gpu.How can i disable the vega 8 so that.i can use my full ram capacity? driver version--radeon 17.7

I also want to know if there is possiblity in future that i can combine the power of Vega 8 and RX560x?I guess this feature was available in previous versions of A series Apu called hybrid crossfire.